Question

3.

A rock sample contains 80g of a potassium-40
(K10) isotope with a half-life of 1.25 billion years.
How much of the potassium-40 isotope will remain
after 2.5 billion years have passed?
A. Og
B. 10g
C. 20g
D. 80g

Answer 1

20 g

Step-by-step explanation:

We are given;

Original mass of K-40 before decay = 80 g

Half life of Potassium-40 = 1.25 billion years

Time taken by the decay = 2.5 billion years

We are required to calculate the mass of K-10 that will remain after 2.5 billion years.

Using the formula;

Remaining mass = Original mass × (1/2)^n

where n is the number of half lives

Number of half lives, n = Time taken ÷ half-life

= 2.5 billion years ÷ 1.25 billion years

= 2

Remaining mass = 80 g × (0.5)^2

= 80 g × 0.25

= 20 g

Therefore, the mass of K-40 isotope that will remain after 2.5 billion years is 20 g.